... also, adding another filter of the same exact color will not gain you anything. The strongest filter (assuming one is stronger than the other) will prevail and the other will do exactly nothing, except to increase your exposure time a little (not as much as Bob suggests above though, since you are filtering already filtered light and the transmission will be close to 100% minus the neutral density characteristics of the filter).

If you need more contrast for a particular negative, try the following:

Try a different paper (I've heard that Fomabrom Variant 111 is more contrasty than other VC papers)
Use the MG grade 5 filter (it may be more contrasty than the magenta in your color head by a bit)
Use a stronger/more contrasty print developer
Bleach back the whites after printing a bit darker

If this is a recurrent problem then:

Develop more to start with
Check your magenta filter to see if it has faded; replacing it with a new/good used filter may help